Orange-fleshed sweet potato-wheat composite breads : physico-chemical, sensory and nutritional quality

Abstract

Vitamin A deficiency (VAD) is a public health problem especially in developing countries. The main strategy that has been used to fight against VAD was the distribution of high dose capsules of vitamin A to people at risk or already affected with VAD. The orange-fleshed cultivars of sweet potato (OFSP) are known to be a good source of provitamin A carotenoids especially β-carotene. However, fresh OFSP roots are perishable but processing them into flour may extend their shelf-life and could be used as an ingredient in baked products such as bread. It is evident that the consumption of bread in Rwanda, for example, is predominantly higher in urban areas than in rural areas. However, the distribution of vitamin A supplements to people at risk is done in both rural and urban areas. Thus, supplementing wheat flour bread with OFSP flour would contribute, to a certain extent, to the eradication of vitamin A deficiency in those areas. The current study aims at exploring the possibility of using OFSP flour in bread-making and the effect of baking on the β-carotene content of OFSP-wheat composite bread and the contribution of the latter to the vitamin A requirements in different groups of people. Substituting wheat flour with OFSP flour increased the time required to reach optimum dough development. It also reduced the tolerance of the dough to mixing. The resistance of the dough to extension increased thereby reducing the extensibility of the dough by 26, 41, 64 and 79% at 10, 20, 30 and 40% substitution levels, respectively. Pasting parameters such as peak, breakdown and final viscosities were decreased probably due to the differences in the physical and chemical properties (granular sistarches from OFSP flour and wheat flour. Bread containing OFSP flour displayed low oven spring, loaf volume and specific volume presumably due to low gluten content. The retention of β-carotene, under baking conditions (190oC for 30 min), significantly differed (p < 0.05) among all bread samples studied. An average of 34, 29 and 17% of β- carotene was lost in breads containing 10, 20 and 30 % OFSP flour, respectively. 9-cis-β- carotene and 13-cis-β-carotene were the main cis isomers of β-carotene formed during baking. Breads containing 10 or 20% OFSP would meet 28.9 and 61% respectively of the recommended dietary allowance (RDA) of vitamin A amongst children aged between 3-10 years. In contrast, the same amount of bread (100 g) supplemented with 30% OFSP flour would provide the amount of vitamin A close to the RDA of vitamin A for children aged between 3-10 years but it would meet less than 50% of the RDA of vitamin A for pregnant and lactating women. The first two principal components (PC1 and PC2) explained about 98% of total variation in the 17 sensory attributes of OFSP-wheat composite breads. PC1 alone accounted for 83.8% of total variation among the breads and separated them based on the amount of OFSP flour they contained. PC2 accounted for 13.9 % of total variation of sensory data. The descriptive sensory panel detected increased sweetness, caramel aroma, sweet potato aroma and flavour in bread samples containing OFSP flour. Breads containing OFSP flour were described as dark (on the crust) probably due to the Maillard reaction and caramelisation. Partial substitution of wheat flour with OFSP flour adversely affected both rheological properties of the dough and its bread-making performance. These effects tended to increase as the amount of OFSP flour increased. Bread containing 10% OFSP flour had oven spring value, loaf volume and specific volume close to the control (but statistically different, p < 0.05) when compared to its counterparts. The descriptive sensory analysis revealed distinct differences in the sensory profiles of OFSP-wheat composite breads as compared with wheat flour bread. The baking process causes detrimental effects to the nutritional quality of OFSP-wheat composite breads through the degradation of â-carotene by heat. Nevertheless, breads containing 20 and 30% OFSP flour were found to possess the potential to fight against vitamin A deficiency in children, pregnant and lactating women. However, it is not yet known how consumers would perceive them, suggesting that a consumer acceptability study may be required.